Process of preventing corrosion of ferrous metal by an aqua ammonia solution



Dec. 22, 1964 L. M. DVORACEK ETAL 3,162,550

PROCESS OF PREVENTING CORROSION 0F FERROUS METAL BY AN AQUA AMMONIA SOLUTION Filed May 8, 1961 [I Pane/2477a CURVE United States Patent 3,162,550 PROCESS OF PREVENTING CORRQSION 0F FER RGUS METAL BY AN AQUA ANMtIvNIA SGLU- TION Louis M. Dvoracek, Brea, and Loren L. Nefir', Fullerton, Califl, assignors to Union Gil Company of California, Los Angeles, Qalifi, a corporation of California Filed May 8, 1961, Ser. No. 108,646 5 Claims. (Cl. 1.48-6.14)

This invention relates to methods of preventing corrosion of ferrous metals which are in contact 'with aqueous solutions of ammonia.

Aqueous solutions of ammonia are produced and handled in large quantities; the major amount being at or near saturation at ambient temperatures and having 25 weight percent dissolved ammonia. As commonly produced and handled, these solutions, commonly referred to as aqua ammonia, also contain a slight amount of dissolved oxygen.

Frequently, however, prolonged storage periods, or possibly bacterial action, reduce the oxygen content of aqua ammonia. We have found that loss or depletion of this dissolved oxygen renders the solutions corrosive to ferrous metals. This corrosive action concentrates at regions of the metal surfaces where the oxide coating or mill scale coating has flaked off or been removed by welding, cold working, etc. 7

Although aqua ammonia is rendered corrosive to ferrous metals by the absence of dissolved oxygen, we have found that introducing oxygen into an oxygen depleted aqua ammonia solution, e.g., by aeration, fails to restore a passive non-corrosive state to the system, but appreciably increases the corrosion rate.

We have found, however, that the aforedescribed corrosion can be arrested by passivating the corroding metal surface with an anodic or chemical passivation treatment. The metal surface is thereafter protected from corrosion by maintaining the aqua ammonia saturated or nearly saturated with oxygen. In this manner, we are ableto restore the system permanently to a passive, non-corrosive state.

We are aware that it has been suggested that corrosion by ammoniacal ammonium nitrate solutions can be prevented by passivating the metal surface andthereafter continuously adding a chemical passivating agent such as a chromate or dichromate. In such a technique, how- 3,1525% Patented Dec. 22, 1964 ice . mersing a cleaned mild steel surface in a 25 weight perever, there occurs a continuous passivation treatment of the metal surface by the chromate or dichromate. This continuous passivation is necessary because the treated metal surfaces do not exist in a stable passive state in j a ammoniacal ammonium nitrate solution, but rapidly revert to an active, corroding state.

In contrast, ferrous metal surfaces in aqua ammonia solutions which are saturated or nearly saturated at ambient conditions with oxygen will remain in a stable pas-' sive state indefinitely. The addition of oxygen to aqua ammonia is not analogous to the prior arts addition of chromate or dichromate to ammonium nitrate solutions because additional of oxygen to the aqua ammoniawithout previously passivating the metal surface will not arrest the corrosion rate, but will actually cause the coran anodic polarization curve and was obtained by cent solution of aqua ammonia and applying a direct current potential between the metal surface and an inert platinum electrode, The aqua ammonia was gently sparged with air to insure that it was saturated with oxygen. The potential of the mild steel surface was observed by a saturated calomel cell connected to the 7 solution by a salt bridge. The applied potential was varied over a range of values and the resultant current flow measured and expressed as current density in microamperes per square centimeter of immersed ferrou metal surface.

As the potential was varied from about --10.00 millivolts, the current density increased until it reached a maximum of about 265 microamperes per square centimeter of immersed metal surface; at the Flade point, F, around 900 millivolts. The terms Flade potential and Blade current are hereinafter used in reference to the voltage and current densities at the Flade point. The Flade point is defined asrthe point of inflection of the anodic polarization curve where any change in the potential, either in the electronegative or electropositive direction, causes a decrease in current density. The current density thereafter rapidly decreased with decreasing negative potential until the current became negative at- 830 millivolts. Finally at about 360 millivolts the overall passive range extended from about -830 to about +600 millivolts. Positive voltages in excess of 6 00 millivolts resulted in a rapid increase in current density. p

Curve 11 illustrates the corrosion rate of the mild steel inthe aqua ammonia solution as a function of the potential of the metal. As the potential is made more positive from +1000 millivolts, the corrosion rate of the metal increasesuntilat the Flade point, P, the corrosion rate at 2 5 C. is at a maximum of about mils per year. The corrosion rate thereafter rapidly decreases until at about-r360 millivolts, the corrosion rate is nil. When a cleaned ferrous metal surface (free of oxides) is immersed in aqua ammonia saturated'with oxygen at ambientcouditionsit will have an initial corrosion rate of about 80,mils per year and a potential of about -900 millivolts. The system rapidly equilibrates to a no current flow condition and reaches the state shown at point A on curve l at 980 millivolts. The corrosion rate at this potential is about 45 mils per year. If the metal surface potential is made more electropositive than the Flade potential 900 millivolts) by a passivating treatment -(hereinaft er described in detail), then the metal will tend to equilibrate to-point Bon curve I at .-360 millivolts. At this point, no corrosion willoccuras'indicated on. curve 11.

The latter condition corresponds to "the most-common occurrence in'handling of aqua ammonia in mild steel equipment which has a mill scale or oxide coating. When oxygen is depleted from the solution, however, the anodic polarizationcurve does notcross the ordinateand a stable passive state such assh'own by point B does notexist;v As aresult, to achievea no current flowconditiom-thefpoten- :tial ofthe metal becomes electronegative, ultimately reacha ing a value of about ,900 to l060millivolts.f This potential corresponds to pointA of curve land represents an active corroding state.-.'.At this potential, areas free of mill scale such as welds or areas where the 'mill scale has been removed by cold working, the ferrous metal will corrode at an initial rate of about 20 mils per, year and an ultimate rate ofabout 5 to 10 mils per year. Upon addition of oxygen to the aqua ammonia without a passivating metal treatment tojreduceits electronegative potential -below thatof the Flade point, the corrosion rate actually increases to about 40 to 80 mils per year. Chemical or anodic passivation of the metal surfaces is therefore necessary prior toaeration of the aqua ammonia to restore the surface to a non-corrosive state.

Various chemical treatments can be employed to passivate the metal surface. In such treatment, the metal vessel is drained of aqua ammonia, cleaned and then washed with aqueous solutions of strong oxidizing agents, e.g., nitric acid of about 10 to 100 weight percent concentration, chromic acid of about to 50 weight percent concentration, 2 percent to saturated solutions of potassium or sodium permanganate, or mixtures thereof. The metals can also be passivated by passing air heated to at least about 75 C. over the surfaces. More suitably, however, the metal surfaces are passivated in situ thus eliminating the need to empty and clean the vessels or tanks, etc., containing the aqua ammonia.

In one technique of in situ passivation, anodic polarization can be used by applying a positive potential to the corroding metal surfaces and immersing a cathode into the aqua ammonia solution. Any convenient source of positive potential can be employed, e.g., an alternating current rectifier, a direct current generator, storage battery, etc. In this method, the potentialof the metal in the aqua ammonia solution is made more electropositive than the Flade potential to the passive range of about -800 to +600 millivolts, preferably about --360 mv. After the passive state is achieved; as readily determined by opening the applied potential circuit and measuring the metal sur face potential to determine if it is about 360 millivolts; the applied voltage is removed and no further treatment will be necessary so long as the aqua ammonia is saturated or nearly saturated with oxygen. The aqua ammonia can be saturated with oxygen by any suitable method such as air blowing the tank contents, circulating a portion through an aerater, etc. This treatment is necessary since if no oxygen is so added, the metal surface reverts to an active state and freely corrodes, requiring the repeated or continuous application of anodic protection. The continuous application has been found to require a current density of between about to 30 microamperes per square centimeter of corroding surface.

In our preferred embodiment, the in situ passivation of the metal surfaces is achieved by the addition of hydrogen peroxide to the aqua ammonia. This treatment is simple, requiring no need for costly electrical installations and thus very suitable in our invention where only a single passivating treatment is necessary. Use of hydrogen peroxide is also advantageous in that it does not introduce any foreign impurities into the aqua ammonia.

The aqua ammonia is preferably treated with hydrogen peroxide at ambient temperatures although slightly altered temperatures from about 5 to about 80 centigrade can be employed. Use of high temperatures is, of course, practical only in closed vessels because of the vapor pressure of ammonia. The hydrogen peroxide is employed-in amounts between about 0.05 and 50 pounds per hundred square feet of surface to be passivated.

After the passivation of the metal surfaces has been completed, as indicated by a potential of the metal surface of about -360 millivolts or by a lack of corrosion, it is necessary only to maintain the aqua ammonia saturated or nearly saturated with oxygen by any suitable method such as those previously described.

Our invention will now be illustrated by the following examples:

Example I A 1020.carbon steel rod was cleaned with hydrochloric acid and immersed in a 25 Weight percent aqueous solution of ammonia which had been depleted of oxygen. The metal had a potential of about 1000 millivolts and was actively corroded by the aqua ammonia at an initial rate of mils per year. The ultimate rate was about 12. mils per year. A similarly cleaned 1020 steel rod was immersed in aqua ammonia which was saturated with oxygen at room temperature. The metal in this solution had a potential of about 980 millivolts, an initial corrosion rate of about mils per year and an ultimate corrosion rate of about 45 mils per year.

A hydrogen peroxide solution (30-35 percent strength) was added in an amount constituting about 0.12 weight percent of the aqua ammonia. The potential of the metal was thereafter measured and found to be -360 millivolts. The metal surface was passive and no further corrosion occurred.

When the oxygen was depleted from the aqua ammonia (by addition of '6 weight percent hydrazine) the metal lost its passivity and was corroded by the aqua ammonia.

When the concentration of aqua ammonia was varied in the experiments no significant changes in the observations occurred.

Example II To demonstrate that mill scale steel can generate a sufiicient cathodic current to activate passive steel surfaces which are in electric conductance therewith, the following experiment was performed:

A 1020 carbon steel rod was cleaned with hydrochloric acid. When immersed in 25% aqua ammonia saturated with dissolved oxygen, the metal had a potential of about 980 millivolts and was actively corroded by the aqua ammonia at an initial rate of 90 mils per year reaching a constant rate of 45 mils per year. Anodic protection of the steel surface was achieved by applying an external positive voltage to the steel rod with a sufiicient current to exceed the Flade value, 265 microamperes per square centimeter. After the potential of the immersed metal had thus been reduced to the stable passive state, about 360 millivolts with no applied voltage, no corrosion Was observed even after extended periods of time.

A steel rod covered with mill scale was immersed in aqua ammonia which had been depleted of dissolved oxygen. The potential of this metal was about 900 to -10,60 millivolts. An electric conductor was placed between the mill scale covered rod and the aforedescribed passivated 1020 steel rod. The steel rod thereupon lost its passivity and was actively corroded by the aqua ammonia.

The preceding examples are intended solely for illustration of various embodiments of my invention; they are are not to be construed as unduly limiting of our invention provided the steps, or obvious equivalents. thereof, set forth by the following claims are followed.

We claim:

1 In a system wherein aferrous metal is in contact with an aqua ammonia solution consisting essentially of ammonia and water and is corroded thereby, the method of arresting said corrosion which comprises the steps of:

(1) passivating said metal by decreasing the electronegati-ve potential of said metal in reference to a saturated calomel electrode to a value more electropositive than the Flade potential of said metal in said solution by applying an .electro-positive .potential to said ferrous metal and supplying an..electrio current thereto at a density of at least about 2.65 microamperes per square centimeter of the surface of said ferrous metal; 7

(2;) discontinuing said passiyating treatment and thereafter preventing said corrosion by introducing oxygen gas into said aqua ammonia solutionjso as to -maintai n said solution saturated with oxygen during the period of said contact.

2. The method of preventing corrosion of ferrous metals by an aqua ammonia solution consisting essentially of water and ammonia that comprises:

( l) subjecting said ferrous metal toa passivating treatment by contact with 'a solution of a strong oxidizing agent to impart to said metal a potential when immersed in said solution that is more electropositive than the Flade potential of said metal insaid solution;

(2) exposing said ferrous metal in said passivated state to contact with said aqua ammonia solution; and

(3) preventing said corrosion during said contact with said aqua ammonia solution by maintaining said aqua ammonia solution saturated with oxygen throughout the period of said contact by the introduction of oxygen gas into said solution.

3. The method of claim 2 wherein said passivating treatment comprises the addition of hydrogen peroxide to said aqua ammonia in amounts between about 0.05 and 50 pounds per hundred square feet of corroding ferrous metal. r

4. The method of claim 2 wherein said aqua ammonia is sparged with an oxygen containing gas.

, References Cited in the file of this patent UNITED STATES PATENTS 1,961,752 Fink et al. June 5, 1934 2,576,680 Guitton Nov. 27, 1951 2,687,994 Russell et al Aug. 31, 1954 3,076,543 McReynolds Feb. 5, 1963 3,078,992 Shapiro Feb. 26, 1963 FOREIGN PATENTS 586,320 Canada Nov. 3, 1959 OTHER REFERENCES Corrosion Handbook, Uhlig (1948), p. 13 6. 

2. THE METHOD OF PREVENTING CORRISON OF FERROUS METALS BY AN AQUA AMMONIA SOLUTION CONSISTING ESSENTIALLY OF WATER AND AMMONIA THAT COMPRISES: (1) SUBJECTING SAID FERROUS METAL TO A PASSIVATING TREATMENT BY CONTACT WITH A SOLUTION OF A STRONG OXIDIZING AGENT TO IMPART TO SAID METAL A POTENTIAL WHEN IMMERSED IN SAID SOLUTION THAT IS MORE ELECTROPOSTIVE THAN THE FLADE POTENTIAL OF SAID METAL IN SAID SOLUTION; (2) EXPOSING SAID FERROUS METAL IN AID PASSIVATED STATE TO CONTACT WITH SAID AQUA AMMONIA SOLUTION; AND (3) PREVENTING SAID CORROSION DURING SAID CONTACT WITH SAID AQUA AMMONIA SOLUTION BY MAINTAINING SAID AQUA AMMONIA SOLUTION SATURATED WITH OXYGEN THROUGHOUT THE PERIOD OF SAID CONTACT BY THE INTRODUCTION OF OXYGEN GAS INTO SAID SOLUTION. 